Nonwovens bonded with acrylamide/N-alkylolacrylamide crosslinking mixture with C2-C10 alkylol

ABSTRACT

An aqueous dispersion includes a self-crosslinking alkylolacrylamide containing interpolymer, the interpolymer being prepared in an emulsion polymerization medium by way of interpolymerizing in the medium: (a) acrylamide monomer; (b) N—C 2  to C 10  alkylolacrylamide monomer, wherein the molar ratio of acrylamide:N—C 2  to C 10  alkylolacrylamide interpolymerized is at least 0.5; and (c) at least one or more additional ethylenically unsaturated monomers added to the medium in an amount of from about 80 weight % to about 98 weight % based on the dry weight of monomer interpolymerized in the medium. The dispersions are particularly useful as binders for fibrous webs.

CLAIM FOR PRIORITY

This application is a divisional application of U.S. patent applicationSer. No. 11/888,164, filed Jul. 31, 2007, entitled, “Self-CrosslinkingDispersions Utilizing Acrylamide/N-Alkylolacrylamide CrosslinkingMixture with C₂-C₁₀ Alkylol”, now U.S. Pat. No. ______. The priority ofU.S. patent application Ser. No. 11/888,164 is hereby claimed and itsdisclosure incorporated herein in its entirety.

TECHNICAL FIELD

The present invention relates to dispersions of self-crosslinkingpolymer dispersions generally and in preferred embodiments to aqueousdispersions of vinyl acetate containing polymers which incorporatemixtures of acrylamide/N-ethylolacrylamide as the crosslinker.

BACKGROUND

Polymeric dispersions are applied to nonwovens to hold together thefibers of a web, such that the web exhibits elevated tensile strength oris self-sustaining. Suitable conventional dispersions includeself-crosslinking polymers of vinyl acetate and ethylene withN-methylolacrylamide (NMA) incorporated into the polymer as acrosslinker. The NMA crosslinker provides enhanced wet strength andresistance to organic solvents; however these compositions tend toevolve formaldehyde which is perceived as a health hazard. Accordingly,numerous attempts have been made to reduce or eliminate formaldehyde inself-crosslinking products.

U.S. Pat. No. 4,449,978 to Iacoviello discloses the usage ofN-methylolacrylamide in conjunction with acrylamide (for example, in a1:1 molar ratio) as the crosslinking chemicals. This approach does notprovide formaldehyde free systems, but it does provide systems withsignificantly lower levels of free formaldehyde compared to systemsusing only N-methylol-acrylamide as the crosslinking chemical. It isseen in this patent that the same amount by weight of the mixtureprovides properties comparable to N-methylolacrylamide only, even thoughonly half of the moles of the mixture contain the methylolself-crosslinking moiety. See Table 4, Col. 13-14.

U.S. Pat. Nos. 3,714,099 and 3,714,100 to Biale and Biale et al.respectively disclose using N-ethylolacrylamide as the crosslinkingmonomer in either vinyl acetate or vinyl acetate/ethylene polymers. U.S.Pat. No. 5,021,529 to Garrett also discloses using N-ethylolacrylamideas an ingredient, or using acrylamide and then adding acetaldehyde tothe finished dispersion polymer thereby making N-ethylolacrylamidein-situ.

While N-ethylolacrylamides have been disclosed in the literature to beuseful as crosslinkers, they are not used commercially because, in part,they do not provide wet strength properties comparable to NMA containingcompositions. See U.S. Pat. No. 5,021,529 to Garrett noted above at Col.9-Col. 10, Tables 1 and 2 as to wet strength of ethyl acrylate binders,where it is seen that the NMA based self-crosslinking resin systemsexhibit much more wet tensile. Thus, N-ethylolacrylamide and higheralkylolacrylamides are generally perceived as unsuitable as acrosslinker for commercial use.

Without intending to be bound by any particular theory, it is noted thatthe equilibrium of the reaction of acrylamide and acetaldehyde lies lesstowards the product, N-ethylolacrylamide, than the correspondingreaction of formaldehyde with acrylamide lies towards its product,N-methylolacrylamide. Therefore, there is inherently a relatively highlevel of acetaldehyde present in N-ethylol-acrylamide compositions.Acetaldehyde (unlike formaldehyde) is a chain transfer agent whichresults in the formation of low molecular weight polymers or oligomersrather than the higher molecular weight polymers needed for EngineeredFabric and textile applications. In these applications, the goal is toachieve infinite molecular weight networks to provide the finishedfabric with wet strength and solvent resistance comparable to wovenfabrics. Since acetaldehyde in the polymerization mixture acts as achain transfer agent, transferring the radical from the chain to amonomer, the resulting product fails to achieve sufficient molecularweight to provide the desired strength properties. Adding moreN-ethylolacryamide will provide a higher level of crosslinking, but alsoincreases the amount of acetaldehyde which has a deleterious effect onmolecular weight.

One proposed method for removing aldehydes from N-alkylolacrylamidemixtures involves reducing the aldehyde to ethanol with sodiumborohydride. See U.S. Pat. No. 5,415,926 to Leighton et al. While thismay be a feasible method of rendering an aldehyde containing mixtureuseable, the method is expensive and time consuming, at best. It mayalso not be effective because the reaction product, N-alkylolacrylamide,may simply undergo retro-reaction to restore an equilibrium compositionwhen used in a binder composition.

SUMMARY OF THE INVENTION

It has been found that adding a mixture of acrylamide andN-alkylolacrylamide (C₂ and higher alkylol) to a binder compositionduring polymerization provides a self-crosslinking dispersion whileameliorating the adverse effects of free aldehyde on the binder polymerso made. Without intending to be bound by theory, it is believed thatthe acrylamide present forces equilibrium towards theN-alkylolacrylamide product, reducing the level of residualalkylaldehyde in the system. This allows higher molecular weightpolymers to be made compared to those systems which only useN-ethylolacrylamide, for example. The acrylamide/alkylolacrylamidemixtures are suitably prepared by using excess acrylamide when preparingalkylolacrylamide.

These new systems provide formaldehyde-free latex polymers which may beused instead of formaldehyde-containing analogs. In this regard, thesame amount by weight of 1:1 N-ethylolacrylamide/acrylamide was usedinstead of 1:1 N-methylolacrylamide/acrylamide in a control polymer. Dueto the difference in molecular weight, there was only roughly 87% of themoles of N-ethylolacrylamide present as the equivalent mass ofN-methylolacrylamide in the control polymer, yet the properties werecomparable. The molecular weight may be incrementally increased byadding an in-situ crosslinker such as diallyl phthalate ortri-allylcyanurate, if so desired. It is also possible to further reducethe excess acetaldehyde to ethanol with the sodium borohydride approachmentioned above or to increase the amount of acrylamide so that theratio is richer in acrylamide in order to further influence equilibriumbetween acrylamide and alkylolacrylamide in the system. Alternatively,one could use relatively less acrylamide and still positively influencethe crosslinking system as compared with using an alkylolacrylamidealone.

Further details and advantages will become apparent from the discussionwhich follows.

DETAILED DESCRIPTION

The invention is described in detail below for purposes of illustrationonly. Modifications within the spirit and scope of the presentinvention, set forth in the appended claims, will be readily apparent toone of skill in the art. Terminology used herein has its ordinarymeaning, unless a more specific definition is expressly provided.

As used in the specification and claims, the term “acrylamide” and“alkylolacrylamide” refers to acrylamide or its functional equivalent,methacrylamide when these terms are in lower case. An initialcapitalization, i.e., “Acrylamide” or “alkylolacrylamide” denotesacrylamide per se.

C₂, C₃, C₄ and so forth refer respectively to two carbon, three carbonand four carbon containing moieties and so on. Thus, an N—C₂ to C₁₀alkylolacrylamide refers to N-ethylolacrylamide, N-propylolacrylamideand so forth up to C₁₀ N-alkylolacrylamides.

“Time delay” and like terminology refers to the total time period aningredient or mixture of ingredients is added to a reaction medium.

The aqueous dispersions described herein may be prepared using materialsand techniques known in the art. Monomer compositions, emulsifiers,testing procedures and so forth appear in U.S. Pat. No. 7,056,847 toWalker et al., the disclosure of which is incorporated herein byreference, as well as the aforementioned U.S. Pat. No. 5,021,529 toGarrett, the disclosure of which is also incorporated by reference.

The compositions of the invention are useful generally as binder resincompositions and are particularly useful in the production of nonwovenproducts made from a web of fibers. The web may be formed by any processknown in the art, such as a carded, air-laid, dry-laid, wet-laid, orair-formed process. The fibers can be natural, synthetic, or a mixturethereof. The binder is applied to the fiber by any means known in theart, such as print, foam, saturate, coating, and spraying; then dried onsteam cans or ovens as currently practiced in the production ofnon-woven rolled goods. Binder add-on levels for nonwovens useful in thepresent invention can be from 0.1 to 100 percent, preferably from 3 to30 percent. Nonwovens made with the binder of the present invention areuseful in applications in which wet integrity or resiliency isimportant, such as wipes, diapers, feminine hygiene, medical, andfiltration products. Non-woven wipes may be used in the dry form andwetted just prior to use, or may be pre-moistened with either aqueous ororganic solvents as known in the art. Wipes are useful in applicationsthat include household cleaning, personal cleansing, baby wipes, andindustrial wipes. Nonwovens of the invention includes both disposablenon-woven products, as well as durable nonwovens such as abrasive pads,medical fabrics, and apparel lining.

The emulsion binder of the invention may also be used as a binder fordouble re-creped paper. Double recreped paper is used in products suchas toweling. The binder is print applied at an add-on level of about 4to 20 percent.

The emulsion binder may be used to bind other fibers, such asfiberglass, and carbon fibers, by means known in the art.

Example 1

The following procedure was used to prepare an aqueous vinyl acetatedispersion of the invention.

480.0 g of deionized water, 3.0 g of Aerosol A-102, 15.00 g of a 25%aqueous solution of sodium vinyl sulfonate (SVS), 0.12 g sodium acetate,3.60 g of Igepal CA-887, 0.6 g of sodium formaldehyde sulfoxylate (SFS),2.40 g of a 1% aqueous solution of Versene 100 and 2.40 g of a 1%aqueous solution of ferrous sulfate was added to a one liter glassreactor. Enough phosphoric acid was provided to adjust the pH to between3.8 and 4.2. While stirring, the vessel was purged with a nitrogen flow.After 10 minutes, the reactor was heated to 65° C. for initiation;however, when the temperature in the reactor reached 50° C., 60.0 g ofvinyl acetate was added to the reactor followed by other ingredients asspecified. When the temperature in the reactor was 58° C. (andthereafter), an aqueous solution of 2.64 g t-butyl hydroperoxide in 60.0g of deionized water was slowly added to the reactor so that the totaldelay time was 4.25 hours. At the same time (and thereafter) an aqueoussolution of 1.72 g of SFS in 54.0 g of deionized water was slowly addedto the reactor so that its total delay time was equivalent to that ofthe aqueous solution of t-butyl hydroperoxide. When the temperature inthe reactor reached 65° C., initiation was observed. Five minutes afterthe observed initiation, an aqueous solution of 72.0 g of deionizedwater, 24.0 g of Aerosol A-102, 8.40 g of Igepal CA-887, 0.60 g ofsodium acetate, 510 g of vinyl acetate, 30.0 g of butyl acrylate and75.0 g of a 48% aqueous solution of acrylamide and N-ethylolacrylamidein a 1:1 molar ratio (NEA) (low acetaldehyde) was added and continued tobe added such that its total delay time was four hours. The temperaturein the reactor was maintained by adjusting the feed rate of the redoxcouple. When the monomer was completely added to the reactor the redoxcouple continued to be added for another five minutes. At this point,0.15 g of t-butyl hydroperoxide was added and the reaction was stirredfor five minutes. Then an aqueous solution of 0.72 g of SFS in 6.0 g ofdeionized water was added and the reaction is stirred for anotherfifteen minutes. The reaction was then allowed to cool. When the reactorhas cooled to 30° C., the dispersion was transferred to a clean quartbottle. The resulting dispersion was 46.29% solids with a pH of 5.0, aBrookfield viscosity of 44 cps, 0.017% grit and a particle size of 343nm.

Example 2

The following procedure was used to prepare an aqueous ethylene/vinylacetate dispersion of the invention.

1610.0 g of deionized water, 14.0 g of a 1% aqueous solution of ferroussulfate, 84.0 g of SVS, 87.5 g Hostapur SAS30, 58.45 g of Polystep TD189, 14.0 g of a 1% aqueous solution of Versene 100, 1.75 g of sodiumsulfate and 1.575 g of sodium erythorbate was added to a ten liter steelreactor. Enough phosphoric acid was added to adjust the pH to 3.8, then2870.0 g of vinyl acetate was added and the reactor was pressurized withethylene to 750 psi. While stirring at 500 rpm, the vessel was allowedto come to equilibrium. Once the reactor was at equilibrium (andthereafter) the agitation was reduced to 400 rpm, an aqueous solution of17.5 g of t-butyl hydroperoxide in 210.0 g of deionized water was slowlyadded along with an aqueous solution of 14.0 g of sodium erythorbate in210.0 g of deionized water. The total delay time for these two slow-addswas about three hours. After initiation (two degree rise in reactortemperature), the reactor jacket was raised to 45° C. and an aqueoussolution of 437.5 g of deionized water, 115.5 g of Hostapur SAS30, 43.75g of Polystep TD 189, 0.525 g of sodium acetate and 483.0 g of a 48%aqueous solution of acrylamide and N-ethylolacrylamide (lowacetaldehyde) in a 1:1 molar ratio was slowly added over four and ½hours. The redox couple slow-adds were also slowed down such that theyfinish 30 minutes after this aqueous delay. The temperature in thereactor was maintained at 60-63° C. while the jacket temperature wasmaintained between 50-52° C. When the redox couple slow adds werefinished, the reactor was allowed to cool to 55° C. Upon cooling, theproduct was transferred to the stripper. An aqueous solution of 3.5 g oft-butyl hydroperoxide in 31.5 g of deionized water was rapidly added tothe reactor. Five minutes later, an aqueous solution of 2.625 g ofsodium erythorbate in 31.5 g of deionized water was added over 15minutes. After stirring overnight, the dispersion was transferred to twoclean gallon bottles. The resulting dispersion was 50.48% solid with apH of 4.7, a Brookfield viscosity of 148 cps, 0.0006% grit and aparticle size of 369 nm.

Examples 3, 4 and Comparative Examples A, B

Following the procedures of Examples 1 and 2, interpolymers weresynthesized using 48% dispersion of 1:1 mixtures of NMA andAcrylonitrile instead of N-ethylolAcrylamide/Acryalmide mixtures. Thus,Composition A was substantially the same as the Example 1 compositionand Composition B was substantially the same as the Example 2composition except for the self-crosslinking chemicals employed.

Compositions of Examples 1, 2 and Compositions A, B were tested for dryand wet tensile by applying a 20% by weight add-on of the interpolymersto Whatman #4 CHR chromatography paper via a saturation process. Thepercent add-on is calculated as follows:

${{add}\text{-}{on}\mspace{14mu} \%} = {\frac{{Saturated}\mspace{14mu} {fiber}\mspace{14mu} {{wt}.{- {Unsaturated}}}\mspace{14mu} {fiber}\mspace{14mu} {{wt}.}}{{Saturated}\mspace{14mu} {fiber}\mspace{14mu} {{wt}.}} \times 100\%}$

The unsaturated fiber weight is the weight of the fibers before anybinder composition is applied. The saturated fiber weight is the weightof the web after the binder has been applied, and the web is dried toremove substantially all of the water. Nonwoven fibrous webs willgenerally have an add-on value of from 2 to 50% by weight, preferablyfrom 15 to 30% by weight in most cases.

After treatment with binder, the paper is then drum dried for 90 secondsat 210 to 215° F. and cured for 2 minutes at 300 to 325° F. 1 inch×5inch strips of the saturated Whatman paper are cut with the 5 inchlength in the cross-machine direction (CMD). Tensile strength ismeasured on a standard Instron tensile tester, set at 3 inch gaugelength and 1 inch per min. crosshead speed. Wet tensile strength ismeasured after soaking specimens for one minute in a 1.0 percentsolution of Aerosol OT wetting agent. 5-7 tensile strips are measuredfor wet tensile strength and an average value is taken. Thecharacteristic wet strength value provided by a binder is reported ingrams per inch. Results appear below in Table 1:

TABLE 1 Tensile Comparison of Examples 1, 2 Compositions and ComparativeExample A, B Compositions CMD CMD Sample Dry Tensile Wet TensileComposition A 6809 g/in 4064 g/in Composition of Example 1 7188 g/in3669 g/in Composition B 7450 g/in 4251 g/in Composition of Example 27274 g/in 3755 g/in

It is seem from the above data that the Example 1 and Example 2Compositions exhibited dry and wet tensile comparable to NMA basedcompositions. These results are surprising because they are contrary toconventional experience where NMA containing compositions generallyexhibit much more tensile, especially wet tensile, thanN-alkylolacrylamide compositions with C₂ and higher alkylol.

There is thus provided in accordance with the invention an aqueousdispersion comprising a self-crosslinking alkylolacrylamide containinginterpolymer, the interpolymer being prepared in an emulsionpolymerization medium by way of interpolymerizing in the medium:acrylamide monomer; N—C₂ to C₁₀ alkylolacrylamide monomer, wherein themolar ratio of acrylamide:N—C₂ to C₁₀ alkylolacrylamide interpolymerizedis at least 0.5; and at least one or more additional ethylenicallyunsaturated monomers added to the medium in an amount of from about 80weight % to about 98 weight % based on the dry weight of monomerinterpolymerized in the medium. Generally, the molar ratio ofacrylamide:N—C₂ to C₁₀ alkylolacrylamide is from 0.5 to 3 such as wherethe molar ratio of acrylamide:N—C2 to C10 alkylolacrylamide is from 0.75to 2. The molar ratio of acrylamide:N—C₂ to C₁₀ alkylolacrylamide may befrom 0.9 to 1.5 or from 1 to 1.25.

The amount of acrylamide and N—C₂ to C₁₀ alkylolacrylamideinterpolymerized in the medium is usually from 2 weight % to 20 weight %based on the dry weight of monomer interpolymerized in the medium andmay be from 3 weight % to 10 weight % based on the dry weight of monomerinterpolymerized in the medium. In one preferred embodiment, the amountof acrylamide and N—C₂ to C₁₀ alkylolacrylamide interpolymerized in themedium is from 4 weight % to 8 weight % based on the dry weight ofmonomer interpolymerized in the medium.

The acrylamide monomer is suitably Acrylamide and the N—C₂ to C₁₀alkylolacrylamide is suitably selected from N-ethylolAcrylamide,N-propylolAcrylamide, N-butylolAcrylamide and N-benzylolAcrylamide intypical cases. The one or more additional ethylenically unsaturatedmonomers are selected from: acrylic acid; acrylonitrile; ethyl acrylate;ethylene; isoprene; methyl methacrylate; methyl styrene; butadiene;propylene; styrene; vinyl esters of versatic acid including VeoVa™ 9,VeoVa™ 10 and VeoVa™ 11 vinyl esters and vinyl acetate.

Veova™ vinyl esters are esters of versatic acid available from HexionSpecialty Chemicals, Columbus, Ohio. The vinyl esters have the followinggeneral structure:

where R₁ and R₂ are alkyl groups which together may collectively containfrom about 6-8 carbon atoms. Especially preferred vinyl esters includeVeoVa™ 9, VeoVa™ 10 and VeoVa™ 11 vinyl esters. In VeoVa™ 9, R₁ and R₂together contain 6 carbon atoms. In VeoVa™ 10, R₁ and R₂ togethercontain 7 carbon atoms. In VeoVa™ 11, R₁ and R₂ together contain 8carbon atoms.

In another aspect of the invention, there is provided an aqueousdispersion comprising a self-crosslinking vinyl acetate andN-ethylolAcrylamide containing interpolymer, the interpolymer beingprepared in an emulsion polymerization medium by way ofinterpolymerizing in the medium: Acrylamide monomer; N-ethylolAcrylamidemonomer, wherein the molar ratio of Acrylamide:N-ethylolAcrylamideinterpolymerized is at least 0.5; and vinyl acetate added to the mediumin an amount of from about 50 weight % to about 98 weight % based on thedry weight of monomer interpolymerized in the medium. These specificpolymers have generally the attributes noed above. The interpolymersuitably contains from about 5% by weight to about 40% by weightethylene repeat units and more typically, from about 15% by weight toabout 30% by weight ethylene repeat units.

Another aspect of the invention is directed to a nonwoven fibroussubstrate comprising fibers bonded with an interpolymer binder, theinterpolymer being prepared in an emulsion polymerization medium by wayof interpolymerizing in the medium: acrylamide monomer; N—C₂ to C₁₀alkylolacrylamide monomer, wherein the molar ratio of acrylamide:N—C₂ toC₁₀ alkylolacrylamide interpolymerized is at least 0.5; and at least oneor more additional ethylenically unsaturated monomers added to themedium in an amount of from about 80 weight % to about 98 weight % basedon the dry weight of monomer interpolymerized in the medium. The fiberscomprise pulp-derived naturally occurring cellulosic papermaking fibersand/or the fibers comprise polyester fibers, polyethylene fibers,polypropylene fibers, nylon fibers or mixtures thereof.

The add-on of interpolymer (dry weight thereof) to the fibrous substrateis from 2% by weight to about 50% and more typically the add-on ofinterpolymer to the substrate is from 15% by weight to about 30%.

While the invention has been described in detail, modifications withinthe spirit and scope of the invention will be readily apparent to thoseof skill in the art. In view of the foregoing discussion, relevantknowledge in the art and references discussed above in connection withthe Background and Detailed Description, the disclosures of which areall incorporated herein by reference, further description is deemedunnecessary.

1. A nonwoven fibrous substrate comprising fibers bonded with aninterpolymer binder, the interpolymer being prepared in an emulsionpolymerization medium by way of interpolymerizing in the medium: (a)acrylamide monomer; (b) N—C₂ to C₁₀ alkylolacrylamide monomer, whereinthe molar ratio of acrylamide:N—C₂ to C₁₀ alkylolacrylamideinterpolymerized is at least 0.5; and (c) at least one or moreadditional ethylenically unsaturated monomers added to the medium in anamount of from about 80 weight % to about 98 weight % based on the dryweight of monomer interpolymerized in the medium.
 2. The nonwovenfibrous substrate according to claim 1, wherein the fibers comprisepulp-derived naturally occurring cellulosic papermaking fibers.
 3. Thenonwoven substrate according to claim 1, wherein the fibers comprisepolyester fibers, polyethylene fibers, polypropylene fibers, nylonfibers or mixtures thereof.
 4. The nonwoven substrate according to claim1, wherein the add-on of interpolymer (dry weight thereof) to thesubstrate is from 2% by weight to about 50%.
 5. The nonwoven substrateaccording to claim 1, wherein the add-on of interpolymer (dry weightthereof) to the substrate is from 15% by weight to about 30%.